FIG. 1 illustrates a conventional transmission line driver 100. The transmission line driver 100 is included in a high-speed serial interface device and has a source coupled logic (SCL) structure. The transmission line driver 100 includes a first transistor 110, a second transistor 115, a current source 120, a first resistor 130, and a second resistor 140 that are connected as shown.
The first transistor 110 is connected to a first power supply terminal and a first output terminal 101 and is switched in response to a first input signal “in” input from a first input terminal. The first transistor 110 is an NMOS transistor and is connected to the first power supply terminal via the current source 120. A ground voltage is applied to the first power supply terminal. A first output signal “out” is output via the first output terminal 101.
The second transistor 115 is connected to the first power supply terminal and a second output terminal 102 and is switched in response to a second input signal “ip” input from a second input terminal. Like the first transistor 110, the second transistor 115 is an NMOS transistor and is connected to the first power supply terminal via the current source 120. A second output signal “outb” is output via the second output terminal 102.
The first resistor 130 is connected to a second power supply terminal Vdd and the first output terminal 101 and the second resistor 140 is connected to the second power supply terminal Vdd and the second output terminal 102. The resistance value of the first and second resistors 130 and 140 is about the same as a terminal resistance value Rt (e.g., 50 ohms) of a differential transmission line connected with the first output terminal 101 and the second output terminal 102.
The first input signal “in” and the second input signal “ip” are generated based on a serial transmission data signal. In a normal mode, the first input signal “in” and the second input signal “ip” have different logic levels; therefore, the first transistor 110 and the second transistor 115 operate complementarily. Accordingly, a differential output voltage of the transmission line driver 100 (i.e., a difference between the first output signal “out” and the second output signal “outb”) is I×(50Ω//50Ω) [V] in the normal mode. “I” is the amount of current flowing in the current source 120. When the differential output voltage is 1000 mV, I=20 [mA]. However, as a power supply voltage (e.g., Vdd) is decreased, influence of power consumption occurring in the transmission line driver 100 is increased. Therefore, it may be desirable to reduce the current “I” flowing in the current source 120, which induces power consumption in the transmission line driver 100.